US consumers spent a record $1.25 billion online on Cyber Monday this holiday season, a 22% increase from the amount spent last year.

With Christmas less than two weeks away, online sales have remained steady, but there are still a few peak online retail sales days left as retailers’ standard shipping deadlines for Christmas delivery will be on December 20.

Additionally, shoppers will have the added advantage of shopping online as late as December 23 as some retailers offer upgraded expedited shipping promotions through that date.

As I mentioned in an earlier Vendor Forum post on APMdigest, website speed and reliability is of critical importance during the online holiday shopping season. Sites that don’t perform well — are slow to load, have periods of unavailability or have inconsistent performance - during these peak periods can negatively impact customers’ experience, their satisfaction and the likelihood that they will continue to spend time on the site or make a purchase. This can mean tens of millions of dollars in lost business with consumers taking their online dollars elsewhere.

Our in-depth assessment of shoppers’ estimated satisfaction with web and mobile site performance for the top US online retailers for the Black Friday through Cyber Monday period found that despite months of preparation by retailers, 86 percent of the top 50 US retailers experienced declines in shoppers’ estimated satisfaction with website performance compared to non-holiday baseline levels.

Additionally, many retailers had web page load times of 10 seconds or more – well beyond the two seconds expected by users. Every additional second of delay can mean millions of dollars in potential revenue lost. Poor performing sites had similar problem patterns in common, including being overloaded with third-party content or heavy on JavaScript and not leveraging browser caching.

Based on our analysis of both the strong and poor performers, below are recommendations for struggling online retailers to improve web and mobile site performance for the remainder of the 2011 holiday season:

Check your third-party content: Third-party content, (e.g. ads, social media plug-ins, images) is necessary, but check on what the impact is and whether there is an alternate solution to embedding it.

Check the content you deliver and control: Check the content size and number of resources by content type. Reduce the size of images where possible. Combine, minify and compress text files such as HTML, JavaScript and CSS files to reduce roundtrips and download time.

Check your JavaScript executions: JavaScript is a big source of performance problems. Use updated code libraries and coding practices. Analyze the impact of JavaScript performance across the major browsers – not just the browsers you use in development.

Check your redirect settings: Many sites still use a series of URL redirects even before the first HTML page is displayed to the user. Proper redirect configuration can save unnecessary roundtrips, eliminate the display of blank browser windows and speed up page load time.

Check your server-side performance: Dynamic pages, e.g. containing location-based deals, your shopping cart or a product search result page, require server-side processing. When servers get overloaded with too many requests and when the application doesn’t scale properly, performance problems cause slow page load and update times.

We’ll continue our assessment of how satisfied shoppers are with Web and mobile site performance for the top US online retailers for the remainder of the holiday season and publish a report on the complete 2011 online holiday shopping season results.

Steve Tack is CTO of Compuware’s Application Performance Management Business Unit.